This invention generally relates to method and apparatus for transferring heat in a fluidized bed and, more particularly, to a contact-charging fluidized bed heat exchanger.
It is known in the prior art that the transfer of heat from a heat source to particles in a fluidized bed is enhanced by application of an alternating electric field across the bed. As described by R. Elsdon and C. J. Shearer in "Heat Transfer in a Gas Fluidized Bed Assisted by an Alternating Electric Field," Chemical Engineering Science, Vol. 32, pp. 1147-1153 (1977), the alternating electric field causes back and forth movement of particles charged naturally by contact electrification in the fluidized bed. In the described apparatus, the bed of particles is rendered fluid by passing an air flow through the bed at a sufficient velocity to cause fluid motion of the particles. A heating element within the bed is grounded, and immersed electrodes alternate in polarity relative to the heating element to cause the particles to move alternately toward the electrodes and toward the heating element. The particles are of a singular type. Though both charge polarities can occur, the predominant particle charge polarity will be determined by the triboelectric nature of the particle type and the materials the particles contact. It is believed that the heat transfer rate is enhanced because driving the particles in this manner causes them to traverse the boundary layer of gas that otherwise surrounds and insulates the element and to contact the heating element. It is also believed that the movement of the particles in the region of the boundary layer acts to stir the gas and erode the boundary layer. As a result of the alternating field, heat transfer rates from the heating element to the particles in the bed are increased.
One use mentioned in the above article for this technique could be for drying paper or textiles. The material to be dried would be passed through the fluidized bed containing heating elements. The material would dry faster than otherwise because of the demonstrated increase in heat transfer rate from the heating element to the particles.
Despite theoretical possibilities, this technique has achieved little commercial use to date. One likely cause is that the heat transfer improvements with a single particle type have been insufficient to justify the use of a high alternating voltage.